This application relates generally to microelectromechanical systems, and more particularly to MEMS devices and methods configured to avoid stiction.
In recent years, increasing emphasis has been made on the development of techniques for producing microscopic systems that may be tailored to have specifically desired electrical and/or mechanical properties. Such systems are generically described as microelectromechanical systems (MEMS) and are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from at least two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in FIGS. 1A-1C illustrating a particular MEMS micromirror structure that may take three positions. Each micromirror includes a reflective surface 116 mounted on a micromirror structural film 112 that is connected by a structural linkage 108 to an underlying substrate 104. Movement of an individual micromirror is controlled by energizing actuators 124a and/or 124b disposed underneath the micromirror on opposite sides of the structural linkage 108. Hard stops 120a and 120b are provided to stop the action of the micromirror structural film 112. Energizing the actuator 124a on the left side of the structural linkage 108 causes the micromirror to tilt on the structural linkage 108 towards that side until one edge of the micromirror structural film 112 contacts the left hard stop 120a, as shown in FIG. 1A. Alternatively, the actuator 124b on the right side of the structural linkage 108 may be energized to cause the micromirror to tilt in the opposite direction, as shown in FIG. 1B. When both actuators are de-energized, as shown in FIG. 1C, the micromirror returns to a static position horizontal to the structural linkage 108. In this way, the micromirror may be moved to any of three positions. This ability to move the micromirror provides a degree of flexibility useful in aligning the MEMS device, although the alignment complexity remains significant. Sometimes hard stops 120a and 120b are not provided so that the micromirror structural film 112 is in direct contact with the substrate 104.
In certain applications, once the micromirror is moved to the proper position, it may remain in that position for ten years or more. Thus, for example, one side of an individual micromirror structural film may remain in contact with the hard stop or substrate for extended periods. Maintaining such contact increases the incidence of dormancy-related stiction. Such stiction results in the micromirror remaining in a tilted position even after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge build up at the junction between the micromirror structural film and the hard stop or substrate. For example, it has been demonstrated that an accumulation of H2O molecules at the junction produces capillary forces that increase the incidence of stiction.
Thus, one solution to overcome stiction is to package the MEMS device in a hermetic or inert environment. Such an environment reduces the possibility of molecule accumulation at the junction. However, such packaging is costly and prone to failure where seals break or are not properly formed. Further, such packaging is incompatible with many types of MEMS devices. In addition, such packaging does not reduce stiction related to charge build up at the junction.
In xe2x80x9cUltrasonic Actuation for MEMS Dormancy-Related Stiction Reductionxe2x80x9d, Proceedings of SPIE Vol. 4180 (2000), which is herein incorporated by reference for all purposes, Ville Kaajakari et al. describe a system for overcoming both molecule and charge related stiction. The system operates by periodically vibrating an entire MEMS device to overcome stiction forces. While there is evidence that vibrating the entire MEMS device can overcome stiction at discrete locations within the device, such vibration causes temporary or even permanent misalignment of the device. Thus, freeing an individual micromirror often requires performance of a costly alignment procedure. Even where the device is not permanently misaligned by the vibration, it is temporarily dysfunctional while the vibration is occurring.
Thus, there exists a need in the art for systems and methods for overcoming stiction in MEMS devices without causing misalignment.
Embodiments of the invention are therefore directed to a microstructure for steering light that mitigates stiction. A substrate is provided on which a structural linkage is connected to support a structural film. The structural film includes a reflective coating. A hold electrode is connected with the substrate at a position laterally beyond an orthogonal projection of the structural film on the substrate. It is configured to hold the structural film electrostatically in a tilted position with respect to the substrate upon application of a potential difference between the structural film and the hold electrode. Because of its positioning with respect to the structural film, it is ensured that the structural film is not in contact with the substrate when the structural film is being held by the hold electrode.
In some embodiments, a snap-in electrode is also provided. The snap-in electrode is connected with the substrate at a position laterally within the orthogonal projection of the structural film on the substrate. It is configured to tilt an end of the structural film in a direction towards the snap-in electrode upon application of a potential difference between the structural film and the snap-in electrode.
The hold electrode may be configured as a comb structure having a plurality of teeth. With such a configuration, a plurality of tilted positions for the structural film may be realized by the application of various potential differences between the structural film and the hold electrode. For example, it may be configured such that an increase in potential difference results in a hold position that deviates more strongly from horizontal.
The microstructure may be configured in different embodiments with a cantilever arrangement or with a torsion-beam arrangement. In embodiments that use the torsion-beam arrangement, a second hold electrode and/or second snap-in electrode may be provided on an opposite side of the structural linkage.
Embodiments of the invention are also directed to a method for fabricating a microstructure for steering light. A structural linkage is formed on a substrate. A structural film is formed on the structural linkage. A reflective coating is deposited on the structural film. A hold electrode is formed on the substrate at a position laterally beyond an orthogonal projection of the structural film on the substrate and configured to hold the structural film electrostatically in a tilted position with respect to the substrate upon application of a potential difference between the structural film and the hold electrode. A snap-in electrode may additionally be formed to tilt the end of the structural film towards the snap-in electrode upon application of a potential difference between the structural film and the snap-in electrode. The hold electrode may be fabricated as a comb structure to permit the selection of a plurality of tilted positions with variation in the potential difference applied. The microstructure may also be fabricated with cantilever or torsion-beam configurations. For embodiments fabricated according to torsion-beam configurations, additional hold and/or snap-in electrodes may be formed on the substrate opposite the structural linkage.
Further embodiments provide a method for operating an optical switch. A first end of a micromirror assembly is tilted towards a substrate by applying a first electrostatic force. Thereafter, the micromirror assembly is held in a first tilted position with respect to the substrate with a second electrostatic force originating from a point laterally beyond an orthogonal projection of the micromirror assembly on the substrate. In one embodiment, the micromirror assembly is released from the first tilted position. Thereafter, a second end of the micromirror assembly is tilted towards the substrate by applying a third electrostatic force. Thereafter, the micromirror assembly is held in a second tilted position with respect to the substrate with a fourth electrostatic force that originates from a point laterally beyond the orthogonal projection of the micromirror assembly on the substrate. In a certain embodiment, the first tilted position is selected from a plurality of possible first tilted positions by establishing a potential difference between the micromirror assembly and a first electrode used to establish the second electrostatic force, and the second tilted position is selected from a plurality of possible second tilted positions by establishing a potential difference between the micromirror assembly and a second electrode used to establish the fourth electrostatic force.
In still other embodiments, a wavelength router is provided that incorporates a microstructure for steering light. The wavelength router is configured for receiving light having a plurality of spectral bands at an input port and for directing subsets of the spectral bands to a plurality of output ports. A free-space optical train is disposed between the input port and the output ports providing optical paths for routing the spectral bands. The optical train also includes a dispersive element disposed to intercept light traveling from the input port. A routing mechanism is provided having at least one dynamically configurable routing element to direct a given spectral band to different output ports. The dynamically configurable routing element includes a micromirror assembly connected with a substrate by a structural linkage. A hold electrode connected with the substrate at a position laterally beyond an orthogonal projection of the micromirror assembly on the substrate is configured to hold the micromirror assembly electrostatically in a first tilted position with respect to the substrate upon application of a potential difference between the micromirror assembly and the hold electrode.